Hexafluoropropylene has been prepared by the pyrolysis of tetrafluoroethylene. This process has several disadvantages. Tetrafluoroethylene, which is itself difficult to prepare and purify, is an explosive compound, which must be stored and handled with the greatest care. The pyrolysis of tetrafluoroethylene inevitably makes some perfluoroisobutylene as a by-product, and this compound is extremely toxic and is costly to remove and destroy.
Another preparative method for hexafluoropropylene is to make it simultaneously with tetrafluoroethylene by pyrolysis of CHClF.sub.2. The product also contains the toxic by-product perfluoroisobutylene, and the process provides a particular mixture of the two products, which may be different from the ratio of products desired by the user. Both of the above synthetic methods are carried out at high temperatures, so it is necessary to make the equipment from rare and expensive metals. Patents describing these processes include U.S. Pat. Nos. 3,873,630, 2,970,176, 3,459,818, 2,758,138, and 3,306,940.
Vapor phase chlorofluorination of two-carbon hydrocarbons to make saturated halocarbons is known, but two-carbon hydrocarbons have only primary carbon-hydrogen bonds, while three-carbon hydrocarbons also have secondary carbon-hydrogen bonds, which have sharply different reactivity in reactions with halogens and halogen compounds.
Vapor phase chlorofluorination of higher hydrocarbons (containing three or more carbon atoms) or isopropyl fluoride is reported in U.S. Pat. Nos. 3,436,430, 3,865,885, and 4,110,406. The reactions described in these references give almost exclusively unsaturated products
U.S Pat. No. 2,900,423 relates to the synthesis of hexafluoropropylene by hydrogenation of CF.sub.3 --CFCl--CF.sub.3 over a catalyst. The patent gives no information about the washing step or the residual K in the catalyst. No information on catalyst life is presented, the longest run lasting only three hours.
Fluorination, (i.e., the reaction of a chlorinated hydrocarbon with HF to substitute F for Cl) is disclosed in U.S. Pat. No. 3,258,500. Transhalogenation (i.e., the exchange of one halogen atom in one compound for a halogen atom in another) is discussed in U.S. Pat. Nos. 3,651,156 and 4,814,522.